1. Field
This invention relates to integrated circuit manufacturing. More particularly, it relates to a method of localized alloying for improved bond reliability, and to bonds made by the method.
2. Related Art
Aluminum bond pads and aluminum-gold wirebonding and ball bonding are ubiquitous in the field of integrated circuit manufacturing. Asymmetric interdiffusion rates, however, often lead to the formation of Kirkendall voids. Coalescence of these voids, driven by the formation of brittle gold-aluminum intermetallic compounds, can increase the resistance and degrade the structural properties of the bond, providing a common mechanism of premature failure of the ball bond. As is the case for many diffusion-driven processes, the rate of growth of the intermetallics, and so of the degradation of desirable bond properties, increases with increasing temperature.
Several approaches have been taken to reduce or prevent the formation of the voids, caused by gold diffusing into aluminum at a higher rate than aluminum diffuses into gold. One such approach is to dope the bulk gold wire with a material such as palladium to effectively decrease the diffusion of gold into aluminum, and so formation of voids. The wire may also be doped with other elements to control grain growth during free air ball formation and strain hardening during loop formation. While doping can facilitate bond formation, the strain hardening tends to improve the stiffness of the wire spans and resistance to wiresweep during molding. Often the alloying process increases the resistivity of the wire. It may also increase the degree of work hardening during loop formation, which may result in minimum loop radii larger than those needed for the small loops in thin mold cap and stacked die applications.
In addition, bulk doping solutions generally have not solved the problem of intermetallic compound-driven bond failure in cases where the ball bond-to-pad interfacial area is small, such as for the ultrafine pitch wirebonds that are becoming increasingly common. In uses where long life and high reliability are required, such as automotive, aerospace, and military applications, or that require operation at high temperatures, premature bond failure presents a significant problem.